Condor 83:34Al Q The Cooper OrnithologicalSociety 1981 PLOT SIZE AS A FACTOR IN WINTER BIRD-POPULATION STUDIES R. TODD ENGSTROM AND FRANCES C. JAMES ABSTRACT.-The influence of plot size on the results of a Winter Bird-Pop- ulation Study was explored by dividing a 58.3-ha (144-acre) area of apparently homogeneous mature pine habitat into nine 6.5-ha subplots. Bird density and species richness were estimated for the nine subplots. To simulate progres- sively larger sample areas, all possible combinations of the subplot survey results were made. Variation in estimates of the density and species richness in the 6.5-ha subplots was large and it decreased as plot size increased. Most of the variation in the distribution of birds among subplots was related to variation in the habitat. In addition to the effects of plot size on census results, the choice of an appropriate size involves consideration of factors such as grid size, sampling speed and time available for censusing. The optimal size in this open pine habitat was between 20 and 25 ha. Plots of this size can be surveyed easily in the early morning hours and would provide estimates of approximately 80% of the species seen on a plot three times as large. In the present study, plots of 6.5 ha provided estimates of only 40% (range 16% to 60%) of the species observed on 58.3 ha. Rarefaction, a statistical technique, is applied here as a method of compar- ing species richness among plots of different size. If one standardizes the number of individuals to the lowest number in a set of plots to be compared, rarefaction can be used to determine the number of species expected in a sample of that size. Avian ecology relies largely on censusing of their effects usually is unknown. Species/ techniques for estimates of community com- area effects are another source of variation position and relative abundance of species. that deserves attention. The number of The standard techniques that are in regular species tends to increase as sampling area use have specific advantages and disadvan- increases. This may be attributable to the tages depending on the objectives of the heterogeneity of the habitat or to the effects study (Jarvinen et al. 1977, Robbins 1978a). of sampling (Connor and McCoy 1979): For regional surveys one might choose the Some studies have been based on com- point count (Jarvinen 1978), frequency sam- parisons among censuses taken on areas of pling (Blonde1 1975), the transect method different size without standardizing them to (Emlen 1977), or the breeding bird survey areas of equal size; others compare cer#sus- (Robbins 1978a). If the objective is to obtain es taken on areas that are too small to sup- detailed quantitative information about the port an adequate representation of the avifauna in a specific habitat, spot-mapping species of birds present. Recommendations on repeated visits probably gives the most for the size of a study area are available: 8- accurate results (Svensson and Williamson 10 ha in wooded habitats (Anon. 1947, Kolb 1969, Robbins 1978a). But even with this 1965, Webster 1966, International Bird intensive method, censuses of breeding Censusing Committee 1970), and 40-100 ha birds (Williams 1936) and wintering popu- in open habitats (IBCC 1970). Most authors lations (Anon. 1947, 1950) as published in agree that, to obtain comparable accuracy, American Birds and elsewhere are subject winter plots should be larger than breeding to sources of error that are difficult to eval- season plots in similar habitats. None of uate. Differences among observers (Enemar these references presents evidence on how et al. 1978), time of day (Robbins 1972, much variation in population estimates is Shields 1977), and season of the year (Jar- attributable to the size of the study plot. vinen et al. 1977) are recognized as possible We report here the results of a Winter biases (Berthold 1976), but the magnitude Bird-Population Study (WBPS) made in an 1341 INFLUENCE OF PLOT SIZE 35 FIGURE 1. The study site is a mature longleaf pine stand near Thomasville, Georgia, having an apparently uniform vegetation stucture. open mature longleaf pine (Pinus palustris) A 58.3-ha (144-acre) square area was divided using forest in southern Georgia in January and a compass and tape measure into nine 6.5-ha (16 acre) subunits. The bird survey was conducted following the February, 1979. We marked the area so that methods described in Audubon Field Notes (Anon. subsamples and combinations of subsam- 1947), Kolh (1965), Robbins (1972) and the IBCC ples could be considered as independent (1970). The results are presented as a WBPS in Amer- surveys for predicting the larger community ican Birds (Engstrom 1980). of birds. The results are used to discuss fac- The size of 6.5 ha for the subplots was selected be- cause it is similar to the 6.1-ha (15 acre) minimum sug- tors involved in the choice of plot size in a gested in Americun Birds for both breeding hird cen- bird survey. suses and winter bird-population studies. The results for these smaller areas were compared and then com- METHODS bined to s emulate’ the results of censuses on succes- sively larger plots. The survey route followed an S- The study site is 2 mi south of Thomasville, Thomas shaped pattern along a grid in which the lines were County, Georgia, on Arcadia Plantation. It is one of the 128 m apart. Care was taken to record hirds crossing few remaining tracts of fully mature longleaf pine in subplot boundaries to minimize duplication. All parts the southeastern United States (Fig. 1). The natural of a subplot were within 64 m of the survey route. occurrence of fire kept large areas of northern Florida There are no special recommendations for the distance and southern Georgia in this type of vegetation in pre- between grid lines for the route in winter bird-popu- historic times (Komarek 1968). This site is now main- lation studies, but the IBCC (1970) recommended 200 tained as a natural area by the Tall Timbers Research m for prairie and tundra and 50 m for closed forest Station and ls burned annually. habitats in the breeding season. C. Rohbins (pers. The forest is composed mostly of trees that are 200, comm.) recommends that the distance between lines and some possibly 300, years old. A narrow strip of should be less in winter studies than in the breeding little-leaf titi (Cyril/a paroifolia) with some sweetgum season because birds of many species are less easily (Liquidambar styraci~ua) and swamp tupelo (Nyssu detected in winter. The walk along the route was timed bi,floru) occurs in a wet area in the center of the site. so that each subplot was completed in 56 minutes. This This titi “stringer” and a few patches of winged sumac was approximately 8.6 mm/ha (3.5 min/acre). Robhins (Rhus copallina) and sapling bitternut hickory (Carya (1972) recommended 12.4 min/ha (5 min/acre) in cordi~ormis) constitute the only shrubby areas on the closed forest habitats. site. The major ground cover species are bracken fern All surveys were started within ?4 minutes of sun- (Pteridium nquilinum), wiregrass (Aristidu strictu), rise, and averaged 3 hours and 45 minutes. A complete and runner oak (Quercus pumilo). Clewell (1978) list- survey ofall 58.3 ha took 8 hours on two days. Although ed many of the herbaceous plants. spreading the survey over two days introduced some 36 R. TODD ENGSTROM AND FRANCES C. JAMES TABLE 1. Survey results for each 6.5-ha subplot. Data are the number of individuals seen on nine surveys. Subplots Species F6 D6 B6 B4 D4 F4 FL? D2 B2 Total* Wood Duck (Aix sponsa) 0 0 0 0 0 0 0 0 2 2 Red-tailed Hawk (Buteo jumuicensis) 0 0 0 0 1 1 1 0 2 Mourning Dove (Zen&u rnacroum) 1 2 0 2 z 0 2 2 7 20 Great Horned Owl (Bubo uirginiunus) 0 0 7 0 0 0 0 0 0 7 Common Flicker (Coluptes auratus) 0 3 3 2 9 0 6 10 4 34 Pileated Woodpecker (Dryocopus pileutus) 2 0 0 1 0 0 0 0 1 4 Red-bellied Woodpecker (Melanerpes curolinus) 5 7 7 5 10 9 17 10 72 Yellow-bellied Sapsucker (Sphyrupicus uurius) 3 3 6 5 3 6 3 z 3 30 Hairy Woodpecker (Picoides uillosus) 0 0 0 0 0 1 0 0 0 1 Downy Woodpecker (P. pubescens) 1 0 0 0 0 1 0 0 1 3 Red-cockaded Woodpecker (P. borealis) 2 15 19 15 20 17 20 16 26 139 Eastern Phoebe (Sayornis phoebe) 0 0 0 0 1 0 0 0 Blue Jay (Cyunocittu cristutu) 0 0 0 3 4 2 4 1 z 1: White-breasted Nuthatch (Sittu curolinensis) 3 7 7 5 17 7 7 9 12 66 Brown-headed Nuthatch (S&u pusillu) 2 14 12 5 7 7 9 10 3 64 House Wren (Troglodytes uedon) 8 5 10 1 11 24 15 9 0 82 Carolina Wren (Thryothorus Zudouiciunus) 3 0 1 0 9 11 7 2 0 33 American Robin (Turdus migrutorius) 3 20 5 16 20 2 1 2 3 72 Eastern Bluebird (Siuliu siulis) 1 6 0 0 1 11 8 0 0 27 Ruby-crowned Kinglet (Regulus culendulu) 0 0 0 0 0 0 1 0 0 1 Loggerhead Shrike (Lanius Zudouiciunus) 0 2 3 1 1 0 1 3 3 13 Solitary Vireo (V. soliturius) 0 3 1 1 2 0 1 0 2 19 Yellow-rumped Warbler (Dendroicu coronutu) 2 0 1 0 0 11 0 0 20 Pine Warbler (D.